1. Technical Field
The present disclosure relates to a laser chamber used in a discharge excited gas laser apparatus.
2. Related Art
The miniaturization and increased levels of integration of semiconductor integrated circuits have led to a demand for increases in the resolutions of semiconductor exposure apparatuses (hereinafter referred to as “exposure apparatus”). Accordingly, advances are being made in the reduction in the wavelengths of light emitted from exposure light sources. Gas laser apparatuses are being used as exposure light sources instead of conventional mercury lamps. At present, a KrF excimer laser apparatus that emits ultraviolet light at a wavelength of 248 nm and an ArF excimer laser apparatus that emits ultraviolet light at a wavelength of 193 nm are being used as gas laser apparatuses for exposure.
In the current generation exposure technology, immersion exposure has been realized, for reducing the apparent wavelength of an exposure light source by filling the space between the projection lens of an exposure apparatus and a wafer with a liquid, and changing the refractive index of the space. In the case where immersion exposure is carried out using an ArF excimer laser apparatus as the exposure light source, the wafer is irradiated with ultraviolet light at a wavelength of 134 nm in water. This technology is referred to as ArF immersion exposure. The ArF immersion exposure may be referred to as ArF immersion lithography.
The spectral line width of the spontaneous oscillation of a KrF or ArF excimer laser apparatus is as wide as 350 to 400 pm. Therefore, chromatic aberration occurs in a laser beam (ultraviolet light) which is reduced and projected by a projection lens of the exposure device onto a wafer, and consequently the resolution is dropped. It is therefore necessary to narrow the spectral line width of the laser beam emitted from the gas laser apparatus to the extent that the chromatic aberration is negligible. The spectral line width may be referred to as “spectral width.” Accordingly, the spectral width has been narrowed by providing a line narrowing module (LNM) having line narrowing elements in a laser resonator of the gas laser apparatus. Here, the line narrowing elements may be, for example, an etalon or a grating. A laser apparatus having a narrowed spectral width in this manner is referred to as a line narrowing laser apparatus.